1. Field of the Invention
The present invention relates to tapes and other materials for joining separate pieces of sheet material along a butt seam. More particularly, the invention relates to making invisible and less visible seams during installation of wall-to-wall carpeting that is stretched (placed in tension) in a direction perpendicular to the seam.
2. Description of the Related Art
There are two primary methods for installing wall-to-wall carpeting, the glue-down method and the stretch-in method. The present invention relates to the stretch-in method of installation, predominately used for residences. The stretch-in method involves fastening the carpet to the floor along one edge of the area to be carpeted, and then stretching the carpet to obtain a smooth, flat carpet surface. The stretched carpet is then fastened to the floor at the opposite edge of the carpeted area, and the process is repeated for the remaining edges. The carpet remains in tension after the installation is completed. A layer of padding material is typically installed under the carpet before it is fastened to the floor. The stretch-in process is intended to remove any wrinkles or unevenness in the carpeting, resulting in a flat, safe, and visually appealing carpet installation.
If, as is often the case, the area to be carpeted is wider than the carpet as produced by the carpet mill, two or more pieces of carpet must be joined along a seam. It is desirable for the seam to conform to the following requirements. First, the seam must be sufficiently strong and durable to maintain its integrity during the life of the carpet, and withstand the tension from the stretch-in installation process. Second, for cosmetic purposes, the seam should be as invisible as possible. Third, the seaming process should be as simple and as economical as possible.
Methods for seaming carpet traditionally included sewing and using various mechanical fasteners, but the predominate method used today employs carpet seaming tape. Carpet seaming tapes are typically comprised of a knitted or woven low-profile scrim of high-strength fiber about three to six inches wide, a strip of paper or foil backing material underneath the scrim, and a layer of thermoplastic resin adhesive overlaying the scrim. To form a seam using the face-seaming process, the two pieces of carpet to be seamed are placed with the tufts facing up and edges abutting over a length of seaming tape. The seaming tape is centered under the two abutting edges with the layer of adhesive facing the carpet backing. A hot seaming iron, or other suitable implement, is used to melt the thermoplastic adhesive and the carpet backing is pushed into the molten thermoplastic adhesive. The adhesive quickly sets, forming a permanent seam. This method is disclosed by, e.g., U.S. Pat. Nos. 3,972,768, 4,097,445 and 4,416,713. An alternate method of seaming, called back seaming, achieves the same result as face seaming but involves placing the carpets face down so that the carpet backing faces up. The seaming tape is placed over the seam with the adhesive towards the carpet backing and heat is applied through the paper backing of the tape. Back seaming is used for constructing decorative carpet panels from many different pieces of carpet whereas face seaming is commonly used for joining large pieces for covering a large area.
Pressure-sensitive adhesives, epoxy adhesives, latex adhesives, and other adhesives may be used to supplement or replace the thermoplastic adhesives on seaming tapes. However, thermoplastic adhesives are widely used because they offer the advantages of moisture resistance, low cost, creep resistance, compatibility with a wide range of carpet backings, repositionability, ease of application, and no exposure to harsh chemicals.
Seam peaking is a near universal problem with seams made using seaming tape in stretched carpet installation. Various seaming tapes and methods have been introduced to minimize seam peaking, some examples of which are described in U.S. Pat. Nos. 4,919,743 (Johnson, et.al., 1990), 5,198,300 (Matthews, et.al., 1993), 5,691,015 (Matthews, 1997), and 5,693,171 (Foster, et.al., 1997). Seam peaking occurs when a taped carpet seam is stretched in a direction perpendicular to the seam. Seam peaking appears as a visible hump or ridge running down the length of the seam. Much to the surprise of many an inexperienced carpet installer, the more an installer stretches the carpet, the more prominent the peaking will become.
There are various theories to explain seam peaking. According to one theory based on classical mechanics, seam peaking is caused by the offset between the tension-carrying plane of the carpet backing and the tension-carrying fibers of the seaming tape. This offset is typically on the order of 0.015 inches to 0.15 inches. Typical seaming tapes and carpet backings, both being relatively flexible, cannot resist the bending moment created by the offset and opposing tensions in the tape and carpet backing. Hence, the backing and tape will bend upwards together until the tension carrying fibers of the carpet backing are more closely in line with the mid-plane of the carpet backing. According to one theory based on classical mechanics, at equilibrium the bending moment created by the offset of the opposing tensile forces will be exactly equal to the reverse bending moment presented by the bending resistance of the carpet backing and seaming tape system as a whole. If the bending resistance of the backing and tape is zero or nearly zero, as appears to be the case for many carpets and tapes, the seam will peak until the offset between opposing tension forces is also zero or near zero. That is, the tension carrying fibers of the seaming tape will rise until they are in line with midplane of tension (also called the shear centroid) in the carpet backing.
Many efforts have been made to prevent seam peaking in carpeting. Johnson '743 and U.S. Pat. No. 4,749,433 (Johnson, et.al., 1988) disclose perhaps the most widely adopted method today, the use of enhanced width seaming tape. However, such tape does not prevent seam peaking, because it is as flexible as ordinary seaming tape. Instead, it appears to operate by mitigating the visual effect of peaking by spreading the peak over a wider strip of tape. The wide tape does not always produce satisfactory results, depending on the type of carpet used, placement of the seam with respect to room lighting, and various other factors. Wide tape is also about twice as expensive as regular tape simply because it consists of about twice as much material for a given length of seam.
Another approach is to adhere the seam to the floor by using a seaming tape with a pressure sensitive adhesive on its reverse side, as disclosed by Foster '171. Most carpet is installed over a foam pad, so this method requires fastening the pad to the floor with nails or staples in the area of the seam to prevent the pad from lifting or expanding upwards under the tape. However, nailing into the floor is often not possible or desirable, and requires additional time and materials. Therefore, this method is not widely used.
Yet another approach has been to use seaming tapes with relatively rigid members, such as steel or high-modulus plastic rods, bars, or wires, placed across the center of the seaming tape. This method is disclosed by Matthews '300. Discrete rigid members are placed on the tape so that they will extend across the seam at regular intervals after the seam is made. A variation on this method, disclosed by Matthews '051, utilizes a single relatively rigid continuous member running down the center of the seaming tape. The center strip also extends across the finished seam. Whether discrete members or a single continuous member are used, the principle of operation is the same: to stiffen the carpet backing and tape system to resist the bending moment created by offset and opposing tension forces. It appears feasible to find commonly available and inexpensive materials that have a sufficiently high bending moment of inertia and also have a thin profile. A thin profile is needed so that the rigid members do not create noticeable lumps or a ridge under the carpet when resting on a typical carpet pad. It has been demonstrated that it is possible to construct tapes with thin rigid members that are practical to use and reduce seam peaking as predicted by the classical mechanical theory of seam peaking explained above.
Unfortunately, it has also been discovered that the use of relatively rigid members across the seam still creates a visible bulge running the length of the seam when the carpet is stretched across the seam. This is true although the seam may be completely flat and invisible while the seamed carpet is in an unstretched state. Thus, it is not the thickness of the rigid members that causes the bulge. Instead, the bulge appears to be caused by another mechanism. Under tension, the rigid members, unlike ordinary seaming tape, do not bend noticeably. However, each rigid member exerts upward pressure on the carpet backing, that results in the profile of the rigid member becoming visible as a bulge on the upper face of the carpet. This mode of bulging is referred to as seam "profiling" because the profile of the rigid member becomes visible.
Profiling can also be explained by classical mechanics, once it is recognized that placing the rigid member across the seam results in a tension across the rigid member. This is because the rigid member is more inelastic, i.e., less able to stretch, than the scrim. Therefore, the tension that would be carried by the scrim in an ordinary tape is transferred to the rigid member. Once the rigid member is in tension, it will not bend noticeably because of its stiffness, but it will transfer its bending moment to the relatively flexible carpet backing. The flexible carpet backing is not able to resist this bending moment and flexes up, allow the rigid member to rise relative to the backing. The rigid member rises in the carpet backing thus reducing the offset in tension forces until equilibrium is reached. At equilibrium, according the classical mechanical theory of seam profiling, the bending moment created by the offset tension forces exactly equals the reverse moment exerted by the carpet and tape on the edges of the rigid member. Meanwhile, the rising rigid member has pushed the carpet backing upwards, causing a bulge to appear on the upper surface of the carpet.
Accordingly, the need exists for a carpet seaming tape that can be used to produce strong, reliable, and permanent seams, able to substantially reduce or eliminate peaking, profiling, and other cosmetic anomalies in stretched carpet. Such a tape should be easy and economical to use with conventional seaming methods such as face seaming using conventional seaming irons.